A hydraulic excavator is provided with an undercarriage capable of traveling by crawlers or the like and a revolving upperstructure swingably mounted on the undercarriage. These undercarriage and revolving upperstructure make up a body. Also provided are attachments such as a boom, arm and bucket for performing digging work or the like and various hydraulic cylinders called a boom cylinder, arm cylinder and bucket cylinder for driving these attachments, respectively. These attachments and hydraulic cylinders make up a front working implement. The front working implement constructed as described above is arranged on the revolving upperstructure to perform various work such as earth/sand digging work.
To drive and control various hydraulic actuators such as the above-described various hydraulic cylinders, this hydraulic excavator is provided with a hydraulic drive system, which is equipped with a main hydraulic pump as a generation source of a hydraulic pressure for feeding pressure oil to the various hydraulic actuators, directional control valves for controlling flows of pressure oil to be fed from the main hydraulic pump to the respective hydraulic actuators, and a working oil reservoir for storing pressure oil to be released from the respective hydraulic actuators via their corresponding directional control valves. When driving a hydraulic cylinder in such a hydraulic drive system, pressure oil is fed from a hydraulic pump to one of a bottom side and rod side of the hydraulic cylinder via a directional control valve and is released from the other side to perform the drive. When plural hydraulic cylinders in a front working implement are operated in combination, pressure oils released from the other sides of the hydraulic cylinders, that is, from their pressure-oil release sides are drained directly to a working oil reservoir without utilizing them.
When a combined operation of boom raising and arm crowding (a combined operation that downwardly pivots the arm while upwardly tilting the boom) is performed by feeding pressure oil to bottom sides of a boom cylinder and arm cylinder and extending them in a hydraulic excavator in a hydraulic excavator, for example, the hydraulic pressure on the pressure-oil feed side, that is, the bottom side of the boom cylinder becomes high, and as a result, the hydraulic pressure on the pressure-oil release side, that is, the rod side of the boom cylinder also rises. However, the hydraulic pressure on the rod side at which such energy still remains has been drained to a working oil reservoir without effectively utilizing it. Paying attention to this problem, the assignee already developed the technology described in Patent Document 1 to effectively utilize the pressure oil on the pressure-oil release side of a hydraulic cylinder, said pressure oil having been effectively unutilized until that time, upon combined operation of hydraulic cylinders.
With a view to facilitating the understanding of the characteristic features of the present invention to be described subsequently herein while positioning as a conventional technology the technology described in Patent Document 1, the details of the conventional technology will be outlined hereinafter. Specifically, a description will be made centering around the technical details of features relevant to the present invention while using the terms employed in Patent Document 1 with reference numerals added in parentheses.
The hydraulic drive system for a hydraulic excavator, said system pertaining to the conventional technology described in Patent Document 1, is provided, according to the description of the embodiments of the invention, with a boom cylinder (6) and arm cylinder (7) arranged for a combined operation as hydraulic actuators of a front working implement and also with a main hydraulic pump (21) commonly employed as a generation source for a hydraulic pressure to be fed to the boom cylinder (6) and arm cylinder (7). To permit driving the boom cylinder (6) and arm cylinder (7) with pressure oil from the common main hydraulic pump (21), parallel lines (27,28) are also provided to feed the pressure oil to both the hydraulic actuators. Also provided are a directional control valve (23) for a boom to control a flow of pressure oil to be fed from the main hydraulic pump (21) to the boom cylinder (6), a directional control valve (24) for an arm to control a flow of pressure oil to be fed from the main hydraulic pump (21) to the arm cylinder (7), and a reservoir line (42) connecting the directional control valve (23) for the boom and the directional control valve (24) for the arm with a working oil reservoir (reservoir (43)).
According to the conventional technology described in Patent Document 1, the above-described hydraulic drive system is provided with a communication line (40) communicating the reservoir line (42) and an upstream side of the directional control valve (24) for the arm with each other and a flow combiner valve (44) arranged in the reservoir line 42 as a closure means for permitting selective closure of the reservoir line (42). This flow combiner valve (44) is a normally-open, hydraulic pilot-operated selector valve which, when the pressure on a bottom side of the arm cylinder (7) has arisen to a high pressure of at least a predetermined pressure value, is switched from an open position to a closed position by the pressure. When this flow combiner valve (44) is in the open position, it serves to return a hydraulic pressure, which is released from the boom cylinder (6), to the working oil reservoir via the directional control valve (23) for the arm. When the pressure on the bottom side of the arm cylinder (7) has arisen to a high pressure of at least the predetermined pressure and the flow combiner valve (44) has been switched to the closed position, on the other hand, it serves to prevent a hydraulic pressure, which is especially on a rod side of the boom cylinder (6), from being returned to the working oil reservoir.
The hydraulic drive system according to the conventional technology is provided with the above-described means so that, when the pressure on the bottom side of the arm cylinder (7) has arisen to a high pressure of at least the predetermined pressure value while earth/sand digging work is performed by extending the boom cylinder (6) and arm cylinder (7) and performing a combined operation of boom raising and arm crowding, the reservoir line (42) is closed by the flow combiner valve (44) and the pressure oil on a rod side of the boom cylinder (6), said pressure oil being released to the reservoir line (42), is guided to the communication line (40) and is fed to the upstream side of the directional control valve (24) for the arm. The pressure oil on the rod side of the boom cylinder (6) is combined to pressure oil, which is to be fed to the arm cylinder (7) from the main hydraulic pump (21), on the upstream side of the directional control valve (24) for the arm, and via the same directional control valve (24), the thus-combined pressure oil is fed to the bottom side of the arm cylinder (7). According to this hydraulic drive system, the arm cylinder (7) can, therefore, be extended faster than before by effectively utilizing the hydraulic pressure on the rod side of the boom cylinder (6), said hydraulic pressure still having residual energy, when a combined operation is performed by the boom cylinder (6) and arm cylinder (7). Patent Document 1: JP-A-2004-346485 (Pages 5-12, FIGS. 1-2)